SEA-SURFACE MEAN-SQUARE SLOPE FROM K(U)-BAND BACKSCATTER DATA

Near-nadir, quasi-specular backscatter data obtained with a 14-GHz airborne radar altimeter are analyzed in terms of the surface mean square slope (mss) parameter. The raw mss data, derived from a least squares fitting of a ray optical scattering model to the return waveform, show an approximately linear wind speed dependence over the wind speed range of 7-15 m s-1, with a slope of about one half that of the optically determined mss. Further analysis based on a simple two-scale scattering model indicates that, at the higher wind speeds, approximately 20% of this apparent slope signal can be attributed to diffraction from waves shorter than the estimated diffraction limit of approximately 0.10 m. The present slope data, as well as slope and other data from a variety of sources, are used to draw inferences on the structure of the high wavenumber portion of the wave spectrum. The data support a directionally integrated model height spectrum consisting of wind speed dependent k-5/2 and classical Phillips' k-3 power law subranges in the range of gravity waves, with a transition between the two subranges occurring around 10 times the peak wavenumber, and a Durden and Vesecky wind speed dependent spectrum in the gravity-capillary wave range. With a nominal value of the spectral constant A(u) = 0.002 in the first k-5/2 subrange, this equilibrium spectrum model predicts a mss wind speed dependence that accords with much of the available data at both microwave and optical frequencies.